A difficult problem in integrated circuit manufacturing is the inability to precisely set the absolute values of passive components such as resistors and capacitors. Good relative precision can be achieved if the passive components are the same type and are on the same integrated circuit, but it has not been possible to reliably achieve a high degree of precision for the absolute values of such passive components.
The variance in the values of passive components from the specified value can be traced to inherent minor variations in the manufacturing process, as, for example, variations in lithography precision, dopant amounts, and oxide thicknesses. The absolute values of these passive components can be estimated during design but cannot be fabricated with precisions using conventional processes.
In some cases, the operating characteristics of a circuit depend on the ratios of the values of passive components, which remain constant if the passive components on such a circuit are affected by similar process variations. Predictable operating characteristics can then be achieved.
There are instances, however, in which the operating characteristics of an integrated circuit are linked to the absolute values of the components, thus requiring high absolute precision for the passive component values. Since it is not possible to obtain precise absolute values for passive components by conventional manufacturing, it would be desirable to be able to trim or adjust the value of the passive components after the circuit has been fabricated.
It is known that a resistor with an adjustable value can be manufactured in the form of a network of resistive elements having metal fuses and shunts. The shunts can be zapping Zener diodes accessible by external circuitry. After measuring the value of the resistor and comparing it with the desired value, the resistor is trimmed or adjusted by melting one or more fuses to increase resistance or by burning out one or more zapping Zener diodes to reduce resistance. Selected fuses can be melted and selected Zener diodes can be burned out by injecting high currents into corresponding circuit pads at the periphery of the chip.
The above described trimming method has various disadvantages which limit its use. First, the intense currents which must be injected into the circuit to create a new connection (for the Zener diodes) or destroy an established connection (for the fuses), may cause unpredictable changes in the rest of the circuit. Second, the new connection formed during the trimming process becomes an integral part of the circuit, often unpredictably altering the circuit's operating characteristics. Third, because the new connections are permanent, procedures must be used to gradually bring the circuit to the desired condition by a sequence of irreversible operations which are particularly slow and expensive. Lastly, it is necessary to provide an extra circuit pad for each trimming element causing an undesirable increase in the size of the integrated circuit chip.